1. Field of the Invention
The present invention relates to a method for producing polybutadiene having enhanced mechanical strength. More particularly the present invention relates to a new method for producing polybutadiene with enhanced mechanical strength in a single polymerization step.
2. Description of the Prior Art
Japanese Examined Patent Publication (Kokoku) No. 49-17666 discloses a process for producing polybutadiene by cis-1,4-polymerizing at least a portion of a feed comprising 1,3-butadiene in an inert organic solvent in the presence of a cis-1,4-polymerization catalyst comprising a cobalt compound and an organic aluminum halide of the formula AlR.sub.n X.sub.3-n, wherein R represents an alkyl radical having 1 to 6 carbon atoms, a phenyl radical, or a cycloalkyl radical, X represents a halogen atom, and n represents a number from 1.5 to 2; then subjecting the resultant polymerization mixture to which may be added in additional amount of 1,3-butadiene and/or an additional amount of the inert organic solvent to 1,2-polymerization in the presence of a 1,2-polymerization catalyst comprising a cobalt compound, an organic aluminum compound of the formula AlR.sub.3 wherein R is the same as defined above, and carbon disulfide.
The above-mentioned process comprises two polymerization steps, cis-1,4-polmerization and 1,2-polymerization and, therefore calls for two polymerization vessels and two sets of temperature-control equipment. The polymerization apparatus required here is therefore expensive, making the polymerization process too costly. Also, the above-mentioned process is disadvantageous in that the yield of polybutadiene per unit time is small.
Japanese Unexamined Patent Publication (Kokai) No. 56-88408 discloses a single step polymerization process for producing polybutadiene. In this type of process, cis-1,4-polymerization and 1,2-polymerization are carried out simultaneously. The 1,3-butadiene is polymerized in a solvent in the presence of a catalyst consisting of a solvent-soluble cobalt compound, a dialkyl aluminum halide of the formula AlR.sub.2 X in an amount of from 1 to 100 moles per mole of the cobalt compond, and carbon disulfide. The polymerization mixture is controlled so that the amount of water in the mixture does not exceed 1 mole per mole of the cobalt compound. According to the disclosure of the unexamined publication, the boiling n-hexane-soluble fraction in the resultant polybutadiene has a small intrinsic viscosity of 0.4, which corresponds to a small molecular weight, and a small content of a cis-1,4-structure. The boiling n-hexane-insoluble fraction of the resultant polybutadiene also exhibits an unsatisfactory melting point of 202.degree. C. or less, even when polymerization is carried out at a low temperature of 20.degree. C.